Content not appearing on single template

I’m having an issue with a website of ours at the moment whereby no content is displaying on the single template (with or without an override). I have a query elsewhere on the site (in the sidebar) which does return the correct query info back, however.

The code in question is below. The query var is used because of a template / permalink arrangement and was working until recently. Is there anything you can think of that could be causing issues on the single template?

The ‘eventbrite_id’ is being pulled in correctly from the query_var and matches that on the Eventbrite site so I know a connection is being made. And of course the sidebar content is returning as intended.

I have some more information. I’ve installed Query Monitor on the site and noticed that the HTTP request to Eventbrite is returning a 404 error with the details below. However if I enter in the OAuth token in the right hand side field it works correctly. Would there be a reason as to why the http request isn’t including the OAuth token?

Strangely, though, these errors are appearing on the sidebar calls which is displaying events correctly.